Aromatic polyesters generally have low melt viscosities or melt strength because of the limits to which their molecular weights can be raised and because many of them are crystalline and have high melting points. Melt processing must be carried out at temperatures at least close to or above these high melting points and viscosities are thereby reduced.
High melt strength is very important for extrusion processing and particularly for extrusion blow molding. This process requires the extrusion of a parison which must be suspended in melt form prior to enclosure in a mold. Most aromatic polyesters lack sufficient melt strength at the processing temperatures which must be used for extrusion blow molding. Commercially available, high molecular weight PET (i.e.poly (ethylene terephthalate)) resins and copolyesters which are either non-crystalline or have lower melting points than PET can be extrusion blow molded, but usually can be used only for making small containers which require less melt strength. The copolymer modifier compositions of this invention increase polyester melt strength and permit even large containers to be extrusion blow molded using only modest modifier concentrations (i.e. from about 1 to about 30%).
In most cases clear blow molded containers are desired, and therefore the copolymer modifier must not reduce the clarity found with amorphous aromatic polyesters. Another object of this invention is to provide a modifier which will not reduce amorphous polyester clarity.
PET generally has an intrinsic viscosity of about 0.5 to 1.1 dl. per gm. and insufficient melt strength for extrusion blow molding applications. Furthermore, PET exhibits a fast rate of crystallization at temperatures above 140.degree. C. which makes the achievement of clear amorphous articles by such thermoplastic fabrication techniques difficult.
Articles produced from PET are usually made by injection-stretch blow-molding techniques in which a parison or preform is injection molded, cooled rapidly and then reheated to a temperature above the T.sub.g (glass transition temperature) but below the crystalline melting point and then blown with stretch orientation to the desired shape. See U.S. Pat. Nos. 3,733,309; 3,745,150; and 3,803,275.
Polyesters that are semicrystalline (e.g. particularly poly(ethylene terephthalate)), are used extensively in many applications that require good solvent resistance and good properties at elevated temperatures. They are frequently processed by injection-stretch blow molding, but there are many components of automobiles and other systems wherein such parts are hollow, and to manufacture these by injection-stretch blow molding is very difficult and expensive. Many such hollow parts can conceivably be made by extrusion-blow molding provided the polymer system has adequate melt strength and viscosity. Unfortunately, polyesters commonly used for injection-stretch blow molding have melt viscosities which are too low to make them suitable for extrusion blow molding. High molecular weight polyesters can be made by solid phase polymerization to make them suitable for extrusion blow molding, but this operation raises the cost of the polyester substantially. Therefore it would be desirable to have extrusion blow moldable polyester compositions made from commercial injection-stretch blow moldable grades of polyesters.
A number of patents deal with polyester melt strength and the extrusion blow molding of polyesters. Some of these teach very high molecular weight polyesters or branched polyesters, others teach copolyesters that are non-crystalline or have reduced melting points, still others describe coextrusion of polyesters with higher viscosity polymers or modification of polyesters with fibers.
There are several patents covering melt strength improvers for polyesters. See U.S. Pat. No. 3,553,157 (Dijkstra et al.), U.S. Pat. No. 4,156,466 (Leslie et al.), U.S. Pat. No. 4,176,101 (Leslie et al.), and U.S. Pat. No. 4,912,167 (Deyrup et al.). Several other patents cover impact modifiers for polyesters which coincidentally improve melt strength. See U.S. Pat. No. 4,034,013 (Lane), U.S. Pat. No. 4,246,378 (Kowetaini et al.), and JP Kokai 90-145011/19. These include core-shell type impact modifiers.
Some of the modifiers described in these references are polymeric. Others are small molecule compounds. Several of these modifiers contain functional groups (e.g. anhydride, isocyanate, epoxy) which are potentially capable of coreacting with the hydroxyl or carboxyl end groups of the polyester. Presumably, these modifiers increase the molecular weight of the polyester by coupling end groups or by forming branches and thereby raise the melt viscosity or melt strength. The low molecular weight modifiers are said to produce clear polyester blends in cases where the polyester is maintained in its amorphous form. Other modifiers appear to contribute varying degrees of haze depending on degree of coreaction and molecular weight.
Since all these modifiers appear to react with polyesters to some degree during melt processing, close control of processing conditions is required to obtain a desired melt strength improvement. Deviation from these conditions can produce insufficient melt strength or in some cases, excessive melt strength. Since the melt strength is dependent on an increase in the effective molecular weight of the polyester, modified resins of this type will be particularly sensitive to hydrolytic degradation which can rapidly reduce molecular weight and melt strength.
For example, in U.S. Pat. No. 3,553,157 (Dijkstra et al.), thick-walled shaped articles which had improved impact strength were prepared from PET and a compound capable of reacting with hydroxyl or carboxyl end groups, for example polyanhydrides. "Thick-walled" is defined by Dijkstra et al. as "shape and/or dimensions are such that they are not readily conducive to orientation of the polymer by drawing." Dijkstra et al. prefer crystalline articles reinforced by glass fibers, and teach nothing with regard to methods of producing extrusion blow-molded articles, blown film or foam from PET, nor anything regarding enhancement of melt characteristics of PET. Dijkstra et al. shows polyfunctional compounds as chain extending agents for poly(alkylene terephthalates).
Further, the addition of an organic sulfonate or an organic sulfonate salt to a mixture of polyester and di- and polyepoxides, and also the addition of ethylene copolymers containing glycidyl groups have been suggested for increasing the melt strength and viscosity of polyesters (see Kometani et al., U.S. Pat. No. 4,246,378). These solutions to the problem have improved polyesters for certain blow molding applications but have proved to be inadequate in providing materials suitable for extrusion blow molding large objects having complex cross-sections such as automobile parts, large bottles and containers.
The object of the present invention is to provide a composition for improving thermoplastic processing and molding of aromatic polyester, such as PET or PET copolyesters, for forming amorphous articles. It is a further object to provide a blend of a melt strength improver and an aromatic polyester for improving thermoplastic processing and molding of aromatic polyesters. A still further object is to provide a clear amorphous extrusion/melt shaped PET or PET copolyester articles. Another object is to provide clear polyester bottles by extrusion blow-molding.